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CELLULAR MOBILE COMMUNICATIONS

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UNIT-I(A) INTRODUCTION TO CELLULAR SYSTEMS

LIMITATIONS OF CONVENTIONAL MOBILE TELEPHONE SYSTEMS

One of many reasons for developing a cellular mobile telephone system and deploying it in many cities is

the operational limitations of conventional mobile telephone systems: limited service capability, poor service

performance, and inefficient frequency spectrum utilization.

LIMITED SERVICE CAPABILITY:

A conventional mobile telephone system is usually designed by selecting one or more channels from a

specific frequency allocation for use in autonomous geographic zones, as shown in Fig.1. The communications

coverage area of each zone is normally planned to be as large as possible, which means that the transmitted power

should be as high as the federal specification allows. The user who starts a call in one zone has to reinitiate the call

when moving into a new zone because the call will be dropped. This is an undesirable radio telephone system since

there is no guarantee that a call can be completed without a handoff capability. The handoff is a process of

automatically changing frequencies as the mobile unit moves into a different frequency zone so that the

conversation can be continued in a new frequency zone without redialing. Another disadvantage of the

conventional system is that the number of active users is limited to the number of channels assigned to a particular

frequency zone.

Fig.1 Conventional Mobile System

POOR SERVICE PERFORMANCE:

In the past, a total of 33 channels were all allocated to three mobile telephone systems: Mobile Telephone

Service (MTS), Improved Mobile Telephone Service (IMTS) MJ systems, and Improved Mobile Telephone Service

(IMTS) MK systems. MTS operates around 40 MHz and MJ operates at 150 MHs; both provide 11 channels; IMTS MK

operates at 450 MHz and provides 12 channels. These 33 channels must cover an area 50 mi in diameter. In 1976,


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New York City had 6 channels of( MJ serving 320 customers, with another 2400 customers on a waiting list. New

York City also had 6 channels of MK serving 225 customers, with another 1300 customers on a waiting list. The large

number of subscribers created a high blocking probability during busy hours. Although service performance was

undesirable, the demand was still great. A high-capacity system for mobile telephones was needed.

Inefficient Frequency Spectrum Utilization:

In a conventional mobile telephone system, the frequency utilization measurement Mo, is defined as the

maximum number of customers that could be served by one channel at the busy hour.

Mo = Number of customers/channel

Mo = 53 for MJ

37 for MK

The offered load can then be obtained by

A = Average calling time (minutes) x total customers / 60 min (Erlangs)

Assume average calling time = 1.76 min.

A1 = 1.76 * 53 * 6 / 60= 9.33 Erlangs (MJ system)

A2 = 1.76 * 37 * 6 / 60= 6.51 Erlangs (MK system)

If the number of channels is 6 and the offered loads are A1 = 9.33 and A2 = 6.51, then from the Erlang B

model the blocking probabilities, B1 = 50 percent (MJ system) and B2 =30 percent (MK system), respectively. It is

likely that half the initiating calls will be blocked in the MJ system, a very high blocking probability. As far as

frequency spectrum utilization is concerned the conventional system does not utilize the spectrum efficiently since

each channel can only serve one customer at a time in a whole area. This is overcome by the new cellular system.

BASIC CELLULAR SYSTEMS

A basic analog cellular system consists of three subsystems: a mobile unit, a cell site, and a mobile telephone

switching office (MTSO), as Fig. 1.1 shows, with connections to link the three subsystems.

Mobile units: A mobile telephone unit contains a control unit, a transceiver, and an antenna system.

Cell site: The cell site provides interface between the MTSO and the mobile units. It has a control unit, radio

cabinets, antennas, a power plant, and data terminals.

MTSO: The switching office, the central coordinating element for all cell sites, con-tains the cellular processor and

cellular switch. It interfaces with telephone company zone offices, controls call processing, provides operation and

maintenance, and han-dles billing activities.

Connections: The radio and high-speed data links connect the three subsystems. Each mobile unit can only use one

channel at a time for its communication link. But the channel is not fixed; it can be any one in the entire band


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assigned by the serving area, with each site having multichannel capabilities that can connect simultaneously to

many mobile units.

The MTSO is the heart of the analog cellular mobile system. Its processor provides central coordination and

cellular administration. The cellular switch, which can be either analog or digital, switches calls to connect mobile

subscribers to other mobile subscribers and to the nationwide telephone network. It uses voice trunks similar to

telephone company interoffice voice trunks. It also contains data links providing supervision links between the

processor and the switch and between the cell sites and the processor. The radio link carries the voice and signaling

between the mobile unit and the cell site. The high-speed data links cannot be transmitted over the standard

telephone trunks and therefore must use either microwave links or T-carriers (wire lines). Microwave radio links or

T-carriers carry both voice and data between cell site and the MTSO.

Fig: Basic cellular system

FIRST, SECOND, THIRD, AND FOURTH GENERATION CELLULAR WIRELESS SYSTEMS

(1G, 2G, 3G AND 4G NETWORKS)

The "G" in wireless networks refers to the "generation" of the underlying wireless network technology.

Technically generations are defined as follows:

1G networks (NMT, C-Nets, AMPS, TACS) are considered to be the first analog cellular systems, which started early

1980s. There were radio telephone systems even before that. 1G networks were conceived and designed purely for

voice calls with almost no consideration of data services


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2G networks (GSM, CDMAOne, D-AMPS) are the first digital cellular systems launched early 1990s, offering

improved sound quality, better security and higher total capacity. GSM supports circuit-switched data (CSD),

allowing users to place dial-up data calls digitally, so that the network's switching station receives actual ones and

zeroes rather than the screech of an analog modem. 2G networks with theoretical data rates up to about 144kbit/s.

3G networks (UMTS FDD and TDD, CDMA2000 1x EVDO, CDMA2000 3x, TD-SCDMA, Arib WCDMA, EDGE, IMT-2000

DECT) are newer cellular networks that have data rates of 384kbit/s and more. The UN's International

Telecommunications Union IMT-2000 standard requires stationary speeds of 2Mbps and mobile speeds of 384kbps

for a 3G.

4G technology refers to the fourth generation of mobile phone communication standards. LTE and WiMAX are

marketed as parts of this generation, even though they fall short of the actual standard.

The ITI has taken ownership of 4G, bundling into a specification known as IMT-Advanced. The document calls for 4G

technologies to deliver downlink speeds of 1Gbps when stationary and 100Mbps when mobile

UNIQUENESS OF MOBILE RADIO ENVIRONMENT

DESCRIPTION OF MOBILE RADIO TRANSMISSION MEDIUM

THE PROPAGATION ATTENUATION.

In general, the propagation path loss increases not only with frequency but also with distance. If the

antenna height at the cell site is 30 to 100 m and at the mobile unit about 3 m above the ground, and the distance

between the cell site and the mobile unit is usually 2 km or more, then the incident angles of both the direct wave

and the reflected wave are very small, as Fig. 2.4 shows. The incident angle of the direct wave is θ1, and the incident

angle of the reflected wave is θ2. θ 1 is also called the elevation angle. The propagation path loss would be 40

dB/dec, 4where “dec” is an abbreviation of decade, i.e ., a period of 10. This means that a 40-dB loss at a signal

receiver will be observed by the mobile unit as it moves from 1 to 10 km. Therefore C is inversely proportional to R.

4

C ∝R−4

= α R−4

(2.3-1)

Where C = received carrier power

R = distance measured from the transmitter to

the receiver α = constant


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FIGURE 2.4 mobile radio transmission models. The difference in power reception at two different distances R1 and R2 will result in

and the decibel expression of Eq. (2.3-2a) is

This 40 dB/dec is the general rule for the mobile radio environment and is easy to remember. It is also easy to

compare to the free-space propagation rule of 20 dB/dec. The linear and decibel scale expressions are

In a real mobile radio environment, the propagation path-loss slope varies as

C ∝R−γ

= α R−γ

(2.3-4)

γ usually lies between 2 and 5 depending on the actual conditions.5 Of course, γ cannot be lower than 2, which is

the free-space condition.

γ The decibel scale expression of Eq. (2.3-4) is

C = 10 log α − 10γ log RdB (2.3-5)

SEVERE FADING. Because the antenna height of the mobile unit is lower than its typical surroundings, and the

carrier frequency wavelength is much less than the sizes of the surrounding structures, multipath waves are

generated. At the mobile unit, the sum of the multipath waves causes a signal-fading phenomenon. The signal


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fluctuates in a range of about 40 dB (10 dB above and 30 dB below the average signal). We can visualize the nulls of

the fluctuation at the baseband at about every half wavelength in space, but all nulls do not occur at the same level,

as Fig. 2.5 shows. If the mobile unit moves fast, the rate of fluctuation is fast. For instance, at 850 MHz, the

wavelength is roughly 0.35 m (1 ft). If the speed of the mobile unit is 24 km/h (15 mi/h), or 6.7 m/s, the rate of

fluctuation of the signal reception at a 10-dB level below the average power of a fading signal is 15 nulls per second

(see Sec. 2.3.3).6

Fig: A typical fading signal while the mobile unit is moving

MODEL OF TRANSMISSION MEDIUM

A mobile radio signal r(t), illustrated in Fig. 2.6, can be artificially characterized5 by two components m(t) and r0(t)

based on natural physical phenomena.

r (t ) = m(t )r0(t) (2.3-6)

The component m(t) is called local mean, long-term fading, or lognormal fading and its variation is due to the

terrain contour between the base station and the mobile unit. The factor r0 is called multipath fading, short-term

fading, or Rayleigh fading and its variation is due to the waves reflected from the surrounding buildings and other structures.

The long-term fading m(t) can be obtained from Eq. (2.3-7a).


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where 2T is the time interval for averaging r(t). T can be determined based on the fading rate of r(t), usually

40 to 80fades.5 Therefore, m(t) is the envelope of r(t), as shown in Fig.2.6a.

Equation (2.3-7a) also can be expressed in spatial scale as

The length of 2L has been determined to be 20 to 40 wavelengths. Using 36 or up to 50 samples in an interval of 40

wavelengths is an adequate averaging process for obtaining the local means.

FIGURE 2.6 A mobile radio signal fading representation. (A) A mobile signal fading.

(B)short-term signal fading.

The factor m(t) or m(x) is also found to be a log-normal distribution based on its char-acteristics caused by the

terrain contour. The short-term fading r0 is obtained by

r0 (in dB) = r (t ) − m(t )dB (2.3-8)

as shown in Fig. 2.6b. The factor r0(t) follows a Rayleigh distribution, assuming that only reflected waves from local

surroundings are the ones received (a normal situation for the mobile radio environment). Therefore, the term

Rayleigh fading is often used.


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DIRECT WAVE PATH, LINE-OF-SIGHT PATH, AND OBSTRUCTIVE PATH

A direct wave path is a path clear from the terrain contour.

The line-of-sight path is a path clear from buildings. In the mobile radio environment, we do not always have a line-

of-sight condition.

When the terrain contour blocks the direct wave path, we call it the obstructive path.

AMPLIFIER NOISE

A mobile radio signal received by a receiving antenna, either at the cell site or at the mobile unit, will be

amplified by an amplifier. We would like to understand how the signal is affected by the amplifier noise. Assume

that the amplifier has an available power gain g and the available noise power at the output is No . The input signal-

to-noise (S/N) ratio is Ps /Ni , the output signal-to-noise ratio is Po /No , and the internal amplifier noise is Nα . Then

the output Po /No becomes

Po g Ps P

s

No = g(Ni ) + Nα = Ni + (Nα /g) (2.3-20) The noise figure F is defined as

maximum possible S/N ratio

F = actual S/N ratio at output (2.3-21) where the maximum possible S/N ratio is measured when the load is an open circuit.

Equation (2.3-21) can be used for obtaining the noise figure of the amplifier.

Ps / kTB No No

F = Po /No = (Po / Ps )kTB= g(kTB) (2.3-22)

Also substituting Eq. (2.3-20) into Eq. (2.3-22) yields

F Ps / kTB Ni + (Nα /g)

(2.3-23)

= Ps /[Ni + (Nα /g)] = kTB

The term kTB is the thermal noise.. The noise figure is a reference measurement between a minimum noise level

due to thermal noise and the noise level generated by both the external and internal noise of an amplifier.

LONG-TERM FADING:

Long-term fading occurs when the propagation environment is changing significantly but this fading is

typically much slower than short-term fading.


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Long-term fading means slower variation in mean signal strength and is produced by movement over much

longer distances.

Long-term is caused by terrain configuration (hill, flat area etc.), which results in local mean attenuation

and fluctuation. Long term fading is also called as slow fading or shadowing.

FACTORS INFLUENCING SHORT TERM FADING:

The following physical factors influence short-term fading in the radio propagation channel:

(1) MULTIPATH PROPAGATION

Multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by

two or more paths. The effects of multipath include constructive and destructive interference, and phase shifting of

the signal.

(2) SPEED OF THE MOBILE

The relative motion between the base station and the mobile results in random frequency modulation due

to different doppler shifts on each of the multipath components.

(3) SPEED OF SURROUNDING OBJECTS

If objects in the radio channel are in motion, they induce a time varying Doppler shift on multipath

components. If the surrounding objects move at a greater rate than the mobile, then this effect dominates fading.

(4) TRANSMISSION BANDWIDTH OF THE SIGNAL

If the transmitted radio signal bandwidth is greater than the “bandwidth" of the multipath channel

(quantified by coherence bandwidth), the received signal will be distorted.

PARAMETERS OF MOBILE MULTIPATH FADING:


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COHERENCE BANDWIDTH:


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Types of small scale fading


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UNIT-I (B)

FUNDAMENTALS OF CELLULAR RADIO SYSTEM DESIGN CONCEPT OF FREQUENCY REUSE CHANNELS: A radio channel consists of a pair of frequencies one for each direction of transmission that is used for full-

duplex operation. Particular radio channels, say F1, used in one geographic zone to call a cell, say C1, with a

coverage radius R can be used in another cell with the same coverage radius at a distance D away.

Frequency reuse is the core concept of the cellular mobile radio system. In this frequency reuse system users

in different geographic locations (different cells) may simultaneously use the same frequency channel (see Fig.1.).

The frequency reuse system can drastically increase the spectrum efficiency, but if the system is not properly

designed, serious interference may occur. Interference due to the common use of the same channel is called co-

channel interference and is our major concern in the concept of frequency reuse.

Fig.1 The ratio of D/R FREQUENCY REUSE SCHEME: The frequency reuse concept can be used in the time domain and the space domain.

Frequency reuse in the time domain results in the occupation of the same frequency in different time slots. It is

called time division multiplexing (TDM). Frequency reuse in the space domain can be divided into two categories.

1. Same frequency assigned in two different geographic areas, such as A.M or FM radio stations using the same

frequency in different cities.

2. Same frequency repeatedly used in a same general area in one system - the scheme is used in cellular systems.

There are many co-channel cells in the system. The total frequency spectrum allocation is divided into K frequency

reuse patterns, as illustrated in Fig. 2 for K — 4, 7, 12, and 19.


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Fig.2 N- cell reuse pattern

Frequency reuses distance:

The minimum distance which allows the same frequency to be reused will depend on many factors, such as

the number of co-channel cells in the vicinity of the center cell, the type of geographical terrain contour, the

antenna height and the transmitted power at each cell site. The frequency reuse distance can be determined from

Where K is the frequency reuse pattern shown in Fig.3, then


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Fig.3.The ratio of D/R

If all the cell sites transmit the same power, then K increases and the frequency reuse distance D increases.

This increased D reduces the chance that cochannel interference may occur.

Theoretically, a large K is desired. However, the total number of allocated channels is fixed. When K is too

large, the number of channels assigned to each of K cells becomes small. It is always true that if the total number of

channels in K cells is divided as K increases, trunking inefficiency results. The same principle applies to spectrum

inefficiency: if the total numbers of channels are divided into two network systems serving in the same area,

spectrum inefficiency increases.

Obtaining the smallest number K involves estimating cochannel interference and selecting the minimum

frequency reuse distance D to reduce cochannel interference. The smallest value of K is K = 3, obtained by setting i =

1, j = 1 in the equation K = i 2 + i j + j

2.

CO-CHANNEL INTERFERENCE REDUCTION FACTOR

Reusing an identical frequency channel in different cells is limited by cochannel interference between cells,

and the cochannel interference can become a major problem.

Assume that the size of all cells is roughly the same. The cell size is determined by the coverage area of the signal

strength in each cell. As long as the cell size is fixed, cochannel interference is independent of the transmitted

power of each cell. It means that the received threshold level at the mobile unit is adjusted to the size of the cell.

Actually, cochannel interference is a function of a parameter q defined as

q = D/R

The parameter q is the cochannel interference reduction factor. When the ratio q increases, cochannel

interference decreases. Furthermore, the separation D is a function of K, and C/I,

D=f(K,C/I)

Where K, is the number of cochannel interfering cells in the first tier and C/I is the received carrier-to-

interference ratio at the desired mobile receiver.


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In a fully equipped hexagonal-shaped cellular system, there are always six cochannel interfering cells in the

fist tier, as shown in Fig.5 ; that is, K = 6. The maximum number of K, in the first tier can be shown as six. Cochannel

interference can be experienced both at the cell site and at mobile units in the center cell. If the interference is

much greater, then the carrier-to-interference ratio C/I at the mobile units caused by the six interfering sites is (on

the average) the same as the C/I received at the center cell site caused by interfering mobile units in the six cells.

According to both the reciprocity theorem and the statistical summation of radio propagation, the two C/I values

can be very close. Assume that the local noise is much less than the interference level and can be neglected. C/I

then can be expressed as

Where is a propagation path-loss slope determined by the actual terrain environment. In a mobile radio

medium,

Usually is assumed to be 4. K is the number of co-channel interfering cells and is equal to 6 in a fully

developed system, as shown in Fig. 5. The six co-channel interfering cells in the second tier cause weaker

interference than those in the first tier. Therefore, the co-channel interference from the second tier of interfering

cells is negligible

Where qk is the cochannel interference reduction factor with Kth co-channel interfering cell

Fig 5: Six effective interfering cells of cell 1


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C/I FOR NORMAL CASE IN AN OMNI DIRECTIONAL ANTENNA SYSTEM.

There are two cases to be considered: (1) the signal and co-channel interference received by the mobile

unit and (2) the signal and co-channel interference received by the cell site. Both cases are shown in Fig.6. Nm and

Nb are the local noises at the mobile unit and the cell site, respectively. Usually Nm and Nb are small and can be

neglected as compared with the interference level. As long as the received carrier-to-interference ratios at both the

mobile unit and the cell site are the same, the system is called a balanced system. In a balanced system, we can

choose either one of the two cases to analyze the system requirement; the results from one case are the same for

the others.

Assume that all Dk are the same for simplicity, then D = Dk and q = qk,

Thus

And

The value of C/I is based on the required system performance and the specified value of is based on the

terrain environment. With given values of C/I and , the co-channel interference reduction factor q can

be determined. Normal cellular practice is to specify C/I to be 18 dB or higher based on subjective tests.

Since a C/I of 18 dB is measured by the acceptance of voice quality from present cellular mobile receivers, this

acceptance implies that both mobile radio multipath fading and co-channel interference become ineffective at that

level. The path-loss slope is equal to about 4 in a mobile radio environment.

The 90th percentile of the total covered area would be achieved by increasing the transmitted power at each cell;

increasing the same amount of transmitted power in each cell does not affect the result. This is because q is not a

function of transmitted power. The factor q can be related to the finite set of cells K in a hexagonal-shaped cellular

system by

Substituting q = 4.41 in above equation yields k=7.


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Fig 6 Cochannel interference from six interferers. (a).receiving at the cell site; (b) receiving at the mobile unit.

TRUNKING AND GRADE OF SERVICE


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DEFINITIONS OF COMMON TERMS USED IN TRUNKING THEORY:

Set-Up Time: the time required to allocate a trunked radio channel to a requesting user.

Blocked Call: Call which cannot be completed at time of request, due to congestion. Also referred to as a lost call.

Holding Time: Average duration of a typical call. Denoted by H(in seconds).

Traffic Intensity: Measure of channel time utilization, which is the average channel occupancy measured in

ERLANGS. This is dimensionless quantity and may be used to measure the time utilization of single or multiple

channels. Denoted by A.

Load: Traffic intensity across the entire trunked radio system, measured in Erlangs.Grade of Service(GOS): A

measure of congestion which is specified as the probability of a call being blocked (for Erlang B), or the probability

of a call being delayed beyond a certain amount of time(for Erlang C).

REQUEST RATE: The average number of call requests per unit time.


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IMPROVING COVERAGE AND CAPACITY IN CELLULAR SYSTEMS

CELL SPLITTING

The motivation behind implementing a cellular mobile system is to improve the utilization of spectrum

efficiency. The frequency reuse scheme is one concept, and cell splitting is another concept. When traffic density

starts to build up and the frequency channels Fi in each cell Ci cannot provide enough mobile calls, the original cell

can be split into smaller cells. Usually the new radius is one-half the original radius. There are two ways of splitting:

In Fig. 8 a, the original cell site is not used, while in Fig. 8 b, it is

New cell radius = Old cell radius/2

Then,

New cell area = Old cell area/4

Let each new cell carry the same maximum traffic load of the old cell , then

New traffic load/Unit area = 4 X Traffic load/Unit area.

There are two kinds of cell-splitting techniques:

1.PERMANENT SPLITTING: The installation of every new split cell has to be planned ahead of time; the number of

channels, the transmitted power, the assigned frequencies, the choosing of the cell-site selection, and the traffic

load consideration should all be considered. When ready, the actual service cut-over should be set at the lowest

traffic point, usually at midnight on a weekend. Hopefully, only a few calls will be dropped because of this cut-over,

assuming that the downtime of the system is within 2h.

2.DYNAMIC SPLITTING: This scheme is based on using the allocated spectrum efficiency in real time. The algorithm

for dynamically splitting cell sites is a tedious job, as we cannot afford to have one single cell unused during cell

splitting at heavy traffic hours.


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Fig.8 Cell splitting

SECTORING

Cell splitting achieves capacity improvement by essentially rescaling the system. By decreasing the cell radius R and

keeping the co-channel reuse ratio D/R unchanged, cell splitting increases the number of channels per unit area.

However, another way to increase capacity is to keep the cell radius unchanged and seek methods to decrease a

D/R ratio . as we now show sectoring increases SIR so that the cluster size may be reduced

In this approach. First the SIR is improved using directional antennas, then capacity improvement is achieved by

reducing the no of cells in a cluster, thus increasing the frequency reuse. However in order to do this successfully. It

is necessary to reduce the relative interference without decreasing the transmitted power.

The co-channel interference in a cellular systems may be decreased by replacing a single Omni directional antenna

at the base station by several directional antennas each radiating within a specified sector. By using directional

antennas, a given cell will receive interference and transmit with only a fraction of the available co-channel cells.

The technique for decreasing co-channel interference and thus increasing system performance by using directional

antennas is called sectoring. The factor by which the co-channel interference is reduced depends on the amount of

sectoring used. A cell is normally partitioned into three 120˚ sectors or six 60˚ sectors as shown in below figure.

Fig: (a) 120˚ sectoring (b) 60˚ sectoring


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MICROCELL ZONE CONCEPT The increased number of handoffs required when sectoring is employed results in an increased load on the

switching and control link elements of the mobile system. A solution to this problem was presented by lee. The

proposal is based on a microcell concept for seven cell reuse as illustrated in fig below. In this scheme, each of the

three (or possibly more) zone sites


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UNIT-II (A)

CO-CHANNEL INTERFERENCE

The frequency-reuse method is useful for increasing the efficiency of spectrum usage but results in cochannel

interference because the same frequency channel is used repeatedly in different cochannel cells. Application of the

cochannel interference reduction factor q= D/R = 4.6 for a seven-cell reuse pattern (K = 7).

In most mobile radio environments, use of a seven-cell reuse pattern is not sufficient to avoid cochannel

interference. Increasing K > 7 would reduce the number of channels per cell, and that would also reduce spectrum

efficiency. Therefore, it might be advisable to retain the same number of radios as the seven-cell system but to

sector the cell radially, as if slicing a pie. This technique would reduce cochannel interference and use channel

sharing and channel borrowing schemes to increase spectrum efficiency.

REAL TIME CO-CHANNEL INTERFERENCE MEASURED AT MOBILE RADIO TRANSCEIVER

When the carriers are angularly modulated by the voice signal and the RF frequency difference between them is

much higher than the fading frequency, measurement of the signal carrier-to-interference ratio C/I reveal that the

signal is

Following kozono and sakamoto s analysis Eq (9.3-6),the term s2(t)+I

2(t) fluctuates close to the fading frequency V/λ

and the term 2S(t)+I(t)cos(Ø1-Ø2)fluctuates to a frequency close to d/dt(Ø1-Ø2).which is much higher than the fading

frequency. Then the two parts of the squared envelope can be separated as


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assume that the random variables s(t),I(t),Ø1,Ø2 are independent ;then the average processes on X and Y are

The signal to Interference ratio Γ becomes

Because X and Y can be separated in Eq.(9.3-6),the preceding computation of Γ in Eq.(9.3-11)could have been

accomplished by means of an envelope detector ,analog to digital converter, and a micro computer. The sampling

delay time Δt should be small enough to satisfy

Determining the delay time Δt to meet the requirement of Eq.(9.3-13) for this calculation is difficult and is a

drawback to this measurement technique. Therefore, real time cochannel interference measurement is difficult to

achieve in practice.

DESIGN OF ANTENNA SYSTEM

DESIGN OF AN OMNIDIRECTIONAL ANTENNA SYSTEM IN THE WORST CASE:

The value of q = 4.6 is valid for a normal interference case in a K=7 cell pattern. In this section we would like to

prove that a K=7 cell pattern does not provide a sufficient frequency re-use distance separation eyen when an ideal

condition of flat terrain is assumed. The worst case is at the location where the weakest signal from its own cell site

but strong interferences from all interfering cell sites. In the worst case the mobile unit is at the cell boundary R, as

shown in Fig. 3. The distances from all six cochannel interfering sites are also shown in the figure: two distances of D

- R, two distances of D, and two distances of D + R.

Following the mobile radio propagation rule of 40 dB/dec, we obtain

Then the carrier-to-interference ratio is


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Fig.3.Cochannel interference (worst case)

Where q=4.6 is derived from the normal case. Substituting q=4.6 into above eqn. we obtain C/I =54 or 17 dB, which

is lower than 18 dB. To be conservative, we may use the shortest distance D – R for all six interferers as a worst

case; then we have

In reality, because of the imperfect site locations and the rolling nature of the terrain configuration, the C/I received

is always worse than 17 dB and could be 14 dB and lower. Such an instance can easily our in a heavy traffic

situation; therefore, the system must be designed around the C/I of the worst case. In that case, a cochannel

interference reduction factor of q=4.6 is insufficient.

Therefore, in an unidirectional-cell system, K = 9 or K 12 would be a correct choice. Then the values of q are

DESIGN OF ANTENNA SYSTEM

Design of a Directional Antenna System:

When the call traffic begins to increase, we need to use the frequency spectrum efficiently and avoid increasing the

number of cells K in a seven-cell frequency reuse pattern. When K increases, the number of frequency channels

assigned in a cell must become smaller (assuming a total allocated channel divided by K) and the efficiency of


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applying the frequency reuse scheme decrease.

Instead of increasing the number K in a set of cells, let us keep K =7 and introduce a directional antenna

arrangement. The cochannel interference can be reduced by using directional antenna. This means that each cell is

divided into three or six sectors and uses three or six directional antennas at a base station. Each sector is assigned

a set of frequencies (channels). The interference between two cochannel cells decreases as shown Fig.4.2

Directional antennas in K=7 cell patterns:

Three sector case: The three-sector case is shown in Fig.4.2. To illustrate the worst case situation, two cochannel

cells are shown in Fig. 4.3(a). The mobile unit at position E will experience greater interference in the lower shaded

cell sector than in the upper shaded cell-sector site. This is because the mobile receiver receives the weakest signal

from its own cell but fairly strong interference from the interfering cell.

In a three-sector case, the interference is effective in only one direction because the front-to-back ratio of a cell-site

directional antenna is at least 10 dB or more in a mobile radio environment. The worst-case cochannel interference

in the directional-antenna sectors in which interference occurs may be calculated. Because of the use of directional

antennas, the number of principal interferers is reduced from six to two (Fig.4.2). The worst case of C/I occurs when

the mobile unit is at position E, at which point the distance between the mobile unit and the two interfering

antennas is roughly D + (R/2); however, C/I can be calculated more precisely as follows. The value of C/I can be

obtained by the following expression (assuming that the worst case is at position E at which the distances from two

interferers are D + 0.7R and D).

Fig.4.2.Interfering cells shown in a seven cell system (two-tiers)


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Fig.4.3. Determination of C/I in a directional antenna system. (a)Worst case in a 120 directional

Antenna system (N=7); (b) worst case in a 60 directional antenna system(N=7)

Let q=4.6; then we have

The C/I received by a mobile unit from the 120° directional antenna sector system expressed in Eq. above greatly

exceeds 18 dB in a worst case. Equation above shows that using directional antenna sectors can improve the signal-

to-interference ratio, that is, reduce the cochannel interference. However, in reality, the C/I could be 6 dB weaker

than in Eq. given above in a heavy traffic area as a result of irregular terrain contour and imperfect site locations.

The remaining 18.5 dB is still adequate.

Six-sector case: We may also divide a cell into six sectors by using six 60°-beam directional antennas as shown in

Fig.4.2. In this case, only one instance of interference can occur in each sector as shown in Fig, 4.2. Therefore, the

carrier-to-interference ratio in this case is which shows a further reduction of cochannel interference. If we use the

same argument as we did for Eq. above and subtract 6 dB from the result of Eq. the remaining 23 dB is still more

than adequate. When heavy traffic occurs, the 60°-sector configuration can be used to reduce cochannel

interference. However, fewer channels are generally allowed in a 60° sector and the trunking efficiency decreases.

In certain cases, more available channels could be assigned in a 60° sector.

Directional antenna in K = 4 cell pattern:

Three-sector case: To obtain the carrier-to-interference ratio, we use the same procedure as in the K = 7 cell-

pattern system. The 120° directional antennas used in the sectors reduced the interferers to two as in K = 7 systems,


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as shown in Fig.4.4.

We can apply Eq. here. For K = 4, the value of q = 3.46; therefore, Eq. becomes

If, using the same reasoning used with Eq. above, 6 dB is subtracted from the result of Eq. above, the remaining 14

dB is unacceptable.

Six-sector case: There is only one interferer at a distance of D + R shown in Fig.4.4. With q=3.46, we can obtain

If 6 dB is subtracted from the above result, the remaining 20 dB is adequate.

Fig. 4.4 Interference with frequency reuse pattern K=4.

Under heavy traffic conditions, there is still a great deal of concern over using a K =4 cell pattern in a 60° sector.

Comparing K =7 and N =4 systems:

A K =7 cell pattern system is a logical way to begin an omnicell system. The co-channel reuse distance is more or less

adequate, according to the designed criterion. When the traffic increases, a three sector system should be

implemented, that is, with three 120° directional antennas in place. In certain hot spots, 60° sectors can be used

locally to increase the channel utilization.

If a given area is covered by both K=7 and K=4 cell patterns and both patterns have a six-sector configuration, then

the K=7 system has a total of 42 sectors, but the K=4 system has a total of only 24 sectors and, of course, the system

of K=7 and six sectors has less cochannel interference.

One advantage of 60° sectors with K=4 is that they require fewer cell sites than 120 sectors with K=7. Two

disadvantages of 60 deg sectors are that (1) they require more antennas to he mounted on the antenna mast and

(2) they often require more frequent handoffs because of the increased chance that the mobile units will travel

across the six sectors of the call. Furthermore, assigning the proper frequency channel to the mobile unit in each

sector is more difficult unless the antenna height at the cell site is increased so that the mobile unit can be located

more precisely. In reality the terrain is not flat, end coverage is never uniformly distributed; in addition, the


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directional antenna front-to-back power ratio in the field is very difficult to predict. In small cells, interference could

become uncontrollable; then the use of a K = 4 pattern with 60 deg sectors in small cells needs to be considered

only for special implementations such an portable cellular systems or narrow beam applications. For small cells, a

better alternative scheme is to use a K =7 pattern with 120° sectors plus the underlay-overlay configuration.

ANTENNA PARAMETERS AND THEIR EFFECTS:

LOWERING THE ANTENNA HEIGHT: Lowering the antenna height does not always reduce the co-channel

interference. In some circumstances, such as on fairly flat ground or in a valley situation, lowering the antenna

height will be very effective for reducing the cochannel and adjacent-channel interference, However, there are

three cases where lowering the antenna height may or may not effectively help reduce the interference.

ON A HIGH HILL OR A HIGH SPOT: The effective antenna height, rather than the actual height, is always considered

in the system design. Therefore, the effective antenna height varies according to the location of the mobile unit.

When the antenna site is on a bill, as shown in Fig. 5.1(a), the effective antenna height is h1 + H.

Fig. 5.1.Lowering the antenna height (a) on a high hill and (b) in a valley

If we reduce the actual antenna height to 0.5h1, the effective antenna height becomes 0.5h1 + H. The reduction in

gain resulting from the height reduction is


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If h1<<H, then the above equation becomes

This simply proves that lowering antenna height on the kill does not reduce the received power at either the cell

site or the mobile unit.

In a valley: The effective antenna height as seen from the mobile unit shown in Fig. 5.1(b) is he1, which is less than

the actual antenna height h1. If he1= 2/3 h1, and the antenna is lowered to ½ h1, then the new effective antenna

height is

Then the antenna gain is reduced by

This simply proves that the lowered antenna height in a valley is very effective in reducing the radiated power in a

distant high elevation area. However, in the area adjacent to the cell-site antenna the effective antenna height is

the same as the actual antenna height. The power reduction caused by decreasing antenna height by half is only

In a forested area: In a forested area, the antenna should clear the tops of any trees in the vicinity, especially when

they are very close to the antenna. In this case decreasing the height of the antenna would not be the proper

procedure for reducing cochannel interference because excessive attenuation of the desired signal would occur in

the vicinity of the antenna and in its cell boundary if the antenna were below the treetop level.

DIVERSITY TECHNIQUES:

SPACE DIVERSITY CONSIDERATIONS

Space diversity, also known as antenna diversity, is one of the most popular forms of diversity used in wireless

systems. Conventional cellular radio systems consist of an elevated base station antenna and a mobile antenna

close to the ground. The existence of a direct path between the transmitter and the receiver is not guaranteed and

the possibility of a number of scatterers in the vicinity of the mobile suggests a Rayleigh fading signal. From this

model jakes deduced that the signals received from spatially separated antennas on the mobile would have

essentially uncorrelated envelopes for antenna separations of one half wavelength or more.

The concept of antenna space diversity is also used in base station design. At each cell site, multiple base

station receiving antennas are used to provide diversity reception. However, since the important scatterers are

generally on the ground in the vicinity of the mobile, the base station antennas must be spaced considerably far


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apart to achieve decorrelation.separations on the order of several tens of wavelengths are required at the base

station .space diversity can thus be used at either the mobile or base station or both .figure shows general block

diagram of a space diversity scheme

Figure 6.12 generalized block diagram for space diversity.

POLARIZATION DIVERSITY:

At the base station, space diversity is considerably less practical than at the mobile because the narrow angle

of incident fields requires large antenna spacings.the comparatively high cost of using space diversity at the base

station prompts the consideration of using orthogonal polarization to exploit polarization diversity .while this only

provides two diversity braches it does allow the antenna elements to be collocated.

In the early days of cellular radio, all subscriber units were mounted in vehicles and used vertical whip antennas.

Today, however, over half of the subscriber units are portable. This means that most subscribers are no longer using

vertical polarization due to hand tilting when the portable cellular phone is used. This recent phenomenon has

sparked interest in polarization diversity at the base station. Measured horizontal and vertical polarization paths

between a mobile and a base station are reported to be uncorrelated by lee and Yeh. The decorrelation for the

signals in each polarization is caused by multiple reflections in the channel between the mobile and base station

antennas. That the reflection coefficient for each polarization is different, which results in different amplitudes and

phases for phases for each, or at least some, of the reflections. After sufficient random reflections, the polarizations

state of the signal will be independent of the transmitted polarization. In practice; however, there is some

dependence of the received polarization on the transmitted polarization.

Circular and linear polarized antennas have been used to characterize multipath inside buildings. When the

path was obstructed, polarization diversity was found to dramatically reduce the multipath delay spread without

significantly decreasing the received power.

FREQUENCY DIVERSITY:

Frequency diversity transmits information on more than one carrier frequency. The rationale behind this technique

is that frequencies separated by more than the coherence bandwidth of the channel will not experience the same

fades. Theoretically, if the channels are uncorrelated, the probability of simultaneous fading wills e the product of

the individual fading probabilities.


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Frequency diversity is often employed in microwave line of sight links which carry several channels in a frequency

division multiplex mode (FDM).due to troposphere propagation and resulting refraction. Deep fading sometimes

occurs. In practice 1: N protection switching is provided by a radio licensee. Where in one frequency is nominally

idle but is available on a standby basis to provide frequency diversity switching for any one of the N other carriers

(frequencies) being used on the same link, each carrying independent traffic. When diversity is needed, the

appropriate traffic is simply switched to the backup frequency. This technique has the disadvantages that it not only

requires spare bandwidth but also requires that there be as many receivers as there are channels used the

frequency diversity. However, for critical traffic, the expense may be justified.

TIME DIVERSITY: Time diversity repeatedly transmits information at time spacing’s that exceed the coherence time

of the channel, so that multiple repetitions of the signal will be received with independent fading conditions,

thereby providing for diversity. One modern implementation of time diversity involves the use of the RAKE receiver

for spread spectrum CDMA,where the multipath channel provides redundancy in the transmitted messages.


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UNIT-2(B) NON CO-CHANNEL INTERFERENCE

ADJUSCENT CHANNEL INTERFERENCE Adjacent channel interference can be eliminated on the basis of the channel assignment, the filter characteristics

and be reduction of near-end-far-end interference. Adjacent channel interference is a board term. It includes next

channel interference (the channel next to the operating channel) and neighboring channel interference (more than

one channel away from the operating channel). Adjacent channel interference can be reduced by the frequency

assignment.

NEXT CHANNEL INTERFERENCE

Next channel interference in an AMPS system affecting a particular mobile unit cannot be caused by transmitters in

the common cell site but must originate at several other cell sites. This is because any channel combiner at the cell

site must combine the selected channels. Normally 21 channels(630 kHz) away, or at least 8 or 10 channels away

from the desired one.therefore,next channel interference will arrive at the mobile unit from other cell sites if the

system is not designed property .also a mobile unit initiating a call on a control channel in cell may cause

interference with the next control channel at another cell site. The methods for reducing this next channel

interference use the receiving end. The channel filter characteristics are a 6dB/oct slope in the voice band and a 24

dB/oct falloff outside the voice band region. If Next channel signal is stringer than 23 Db, it Will interfere with the

desired signal. The filter with a sharp falloff slope can help to reduce all the adjacent channel interference, including

the next channel interference. The same consideration is applied to digital systems.


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NEIGHBORING CHANNEL INTERFERENCE The channels that are several channels away from next channel will cause interference with the desired signal.

Usually, a fixed set of serving channels is assigned to each cell site. If all the channels are simultaneously transmitted

at one cell site antenna; a sufficient amount of band isolation between channels is required for a multichannel

combiner to reduce products. This requirement is no different from other non mobile radio systems. Assume that

band separation requirements can be resolved, for example, by using multiple antennas instead of one antenna at

the cell site. There will be no intermodulation products. A truly linear broadband amplifier can realize this idea.

However, it is a new evolving technology.

Another type of adjacent channel interference is unique to the mobile radio system. In the mobile radio system,

most mobile units are in motion simultaneously. Their relative positions change from time to time. In principle, the

optimum channel assignments that avoid adjacent channel interference must also change from time to time .one

unique station that causes adjacent channel interference in mobile radio system.

TRANSMITTING AND RECEIVING CHANNELS INTERFERENCE

In FDMA and TDMA systems, the transmitting channels and receiving channels have to be separated by a guard

band mostly 29MHz.it is because the transmitting channels are so strong that they can mask the weak signals

received from receiving channels. The duplexer can only provide 30 dB to 40dB isolation. The band isolation is the

other means to reduce the interference.

INTERFERENCE FROM ADJACENT SYSTEMS

The frequency bands allocated between AMPS and iDEN in 800 MHz systems are shown in Fig 10.4 in 1993. IDEN

transmitted in the band 851-866 MHz , using several broad band amplifiers to cover this band. The IM(2A-

B)generated from the nonlinear amplifier interfered with the cellular base received signals. Then , the broadband

amplifiers were removed.


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UNIT-III

CELL COVERAGE FOR SIGNALAND TRAFFIC SIGNAL REFLECTIONS IN FLAT AND HILLY TERRAIN

The ground incident angle and the ground elevation angle over a communication link are described as follows. The

ground incident angle 0 is the angle of wave arrival incidentally pointing to the ground as shown in Fig. 1.1. The

ground elevation angle is the angle of wave arrival at the mobile unit as shown in Fig. 1.1

Figure 1.1 Representation of Ground Incident Angle θ and Ground Elevation Angle φ

Based on Snell’s law, the reflection angle and incident angle are the same. Since in

graphical display we usually exaggerate the hilly slope and the incident angle by enlarging the vertical scale, as

shown in Fig. 1.2, then as long as the actual hilly slope is less than 100, the reflection point on a hilly slope can be

obtained by following the same method as if the reflection point were on flat ground. Be sure that the two

antennas (base and mobile) have been placed vertically, not perpendicular to the sloped ground. The reason is that

the actual slope of the hill is usually very small and the vertical stands for two antennas are correct. The scale

drawing in Fig. 1.2 is somewhat misleading however, it provides a clear view of the situation.

Fig 1.2 Ground reflection angle and reflection point


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PHASE DIFFERENCE BETWEEN THE DIRECT PATH AND THE REFLECTED PATH

Based on a direct path and a ground reflected path, the equation

Indicates a two-wave model which is used to understand the path-loss phenomenon in a mobile radio

environment. It is not the model for analyzing the multipath fading phenomenon. In a mobile environment av = -

1 because of the small incident angle of the ground wave caused by a relatively low cell-site antenna height. Thus,

, then

, thus


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Where P is the power difference in decibels between two different path lengths and G is the gain (or loss) in

decibels obtained from two different antenna heights at the cell site. From these measurements, the gain from a

mobile antenna height is only 3 dB/oct, which is different from the 6 dB/oct . Then

CONSTANT STANDARD DEVIATION ALONG A PATH-LOSS SLOPE

When plotting signal strengths at any given radio-path distance, the deviation from predicted value. is

approximately 8 dB.1012 This standard deviation of 8 dB is roughly true in many different areas. The explanation is

as follows. When a line-of-sight path exists, both the direct wave path and reflected wave path are created and are

strong. When an out-of-sight path exists, both the direct wave path and the reflected wave path are weak. In

either case, according to the theoretical model, the 40-dB/dec path-loss slope applies. The difference between

these two conditions is the 1-mi intercept (or 1-km intercept) point. It can be seen that in the open area, the 1-mi

intercept is high. In the urban area, the 1-mi intercept is low. The standard deviation obtained from the measured

data remains the same along the different path-loss curves regardless of environment.

Support for the above argument can also be found from the observation that the standard deviation

obtained from the measured data along the predicted path-loss curve is approximately 8 dB. The explanation is

that at a distance from the cell site, some mobile unit radio paths are line-of-sight, some are partial line-of-sight,

and some are out-of-sight. Thus the received signals are strong, normal, and weak, respectively. At any distance,

the above situations prevail. If the standard deviation is 8 dB at one radio-path distance, the same 8dB will be

found at any distance. Therefore a standard deviation of 8 dB is always found along the radio path as shown in

Fig.3

The standard deviation of 8 dB from the measured data near the cell site is due mainly to the close-in

buildings around the cell site. The same standard deviation from the measured data at distant locations is due to

the great variation along different around the cell site. The same standard deviation from the measured data at a

distant location is due to the great variation along different radio paths.


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Fig 3 An 8-dB local mean spread

MERITS OF POINT-TO-POINT MODEL

The area-to-area model usually only provides an accuracy of prediction with a standard deviation of 8 dB,

which means that 68 percent of the actual path-loss data are within the ± 8 dB of the predicted value. The

uncertainty range is too large. The point-to-point model reduces the uncertainty range by including the detailed

terrain contour information in the path-loss predictions.

The differences between the predicted values and the measured ones for the point-to-point model were

determined in many areas. In the following discussion, we compare the differences shown in the Whippany, N.J.,

area and the Camden- Philadelphia area. First, we plot the points with predicted values at the x-axis and the

measured values at the y-axis, shown in Fig. 4. The 450 line is the line of prediction without error. The dots are

data from the Whippany area, and the crosses are data from the Camden-Philadelphia area. Most of them, except

the one at 9 dB, are close to the line of prediction without error.

The mean value of all the data is right on the line of prediction without error. The standard deviation of

the predicted value of 0.8 dB from the measured one.

In other areas, the differences were slightly larger. However, the standard deviation of the predicted

value never exceeds the measured one by more than 3 dB. The standard deviation range is much reduced as

compared with the maximum of 8 dB from area-to-area models. The point-to-point model is very useful for

designing a mobile cellular system with a radius for each cell of 10 mi or less. Because the data follow the log-

normal distribution, 68 percent of predicted values obtained from a point-to-point prediction model are within 2

to 3 dB. This point-to-point prediction can be used to provide overall coverage of all cell sites and to avoid co-


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channel interference. Moreover, the occurrence of handoff in the cellular system can be predicted more

accurately.

The point-to-point prediction model is a basic tool that is used to generate a signal coverage map, an

interference area map, a handoff occurrence map, or an optimum system design configuration, to name a few

applications.

Fig.4. Indication of errors in point-to-point predictions under non obstructive conditions.

FOLIAGE LOSS Foliage loss is a very complicated topic that has many parameters and variations. The sizes of leaves,

branches, and trunks, the density and distribution of leaves, branches, and trunks, and the height of the trees

relative to the antenna heights all be considered. An illustration of this prv1em is shown in Fig. 5.1. There are three

levels: trunks, branches, and leaves. In each level, there is a distribution of sizes of trunks, branches, and leaves

and also of the density and spacing between adjacent trunks, branches, and leaves. The texture and thickness of

the leaves also count. This unique problem can become very complicated and is beyond the scope of this book. For

a system design, the estimate of the signal reception due to foliage loss does not need any degree of accuracy.

Furthermore, some trees, such as maple or oak, lose their leaves in winter, while others, such as pine, never

do. For example, in Atlanta, Georgia, there are oak, maple, and pine trees. In summer the foliage is very heavy, but

in winter the leaves of the oak and maple trees fall and the pine leaves stay. In addition, when the length of pine

needles reaches approximately 6 in., which is the half wavelength at 800 MHz, a great deal of energy can be

absorbed by the pine trees. In these situations, it is very hard to predict the actual foliage loss.


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However, a rough estimate should be sufficient for the purpose of system design. In tropic zones, the sizes of

tree leaves are so large and thick that the signal can hardly penetrate. In this case, the signal will propagate from

the top of the tree and deflect to the mobile receiver. We will include this calculation also.

Sometime the foliage loss can be treated as a wire-line loss, in decibels per foot or decibels per meter,

when the foliage is uniformly heavy and the path lengths are short. When the path length is long and the foliage is

non uniform, then decibels per octaves or decibels per decade are used. In general, foliage lose occurs with respect

to the frequency to the fourth power. Also, at 800 MHz the foliage Lou along the radio path is 40 dB/dec, which is

20 dB more than the free- space loss, with the same amount of additional loss for mobile communications.

Therefore, if the situation involves both foliage loss and mobile communications, the total loss would be 60 dB/dec

(=20 dB/ dec of free-space loss + additional 20 dB due to foliage loss + additional 20 dB due to mobile

communication).

Fig.5.1. A characteristic of foliage environment

This situation would be the case if the foliage would line up along the radio path. A foliage loss in a suburban area

of 58.4 dB/dec is shown in Fig.5.2. As demonstrated from the above two examples, close-in foliage at the

transmitter site always heavily attenuates signal reception. Therefore, the cell site should be placed away from

trees.


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Fig.5.2. Foliage loss calculation in suburban areas

SMALL SCALE MULTIPATH PROPAGATION The multipath propagation of radio signals over a short period of time or to travel a distance is considered to be

the small scale multipath propagation. As every type of multipath propagation results in generating a faded signal

at receiver, the small scale multipath propagation also results in small scale fading. Hence, the signal at the

receiver is obtained by combining the various multipath waves. These waves will vary widely in amplitude and

phase depending on the distribution of the intensity and relative propagation time of the waves and bandwidth of

the transmitted signal.

The three fading effects that are generally observed due to the small scale multipath propagation are,

1. Fast variations in signal strength of the transmitted signal for a lesser distance or time interval.

2. The variations in Doppler shift on various multipath signals are responsible for random frequency

modulation

3. The time dispersed signals are resulted due to multipath propagation delays.

In order to determine the small scale fading effects, we employ certain techniques. They are,

D Direct RF pulse measurement

E Spread spectrum sliding correlation measurement.

F Swept frequency measurement.

The first technique provides a local average power delay profile.

The second technique detects the transmitted signal with the help of a narrow band receiver preceded by a

wide band mixer though the probing (or received) signal is wide band.


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The third technique is helpful in finding the impulse response of the channel in frequency domain. By knowing

the impulse response we can easily predict the signal obtained at the receiver from the transmitter.

G Direct RF pulse measurement H Spread spectrum sliding correlation measurement. I Swept frequency measurement.

The first technique provides a local average power delay profile. The second technique detects the transmitted signal with the help of a narrow band receiver preceded by a wide

band mixer though the probing (or received) signal is wide band.

The third technique is helpful in finding the impulse response of the channel in frequency domain. By knowing the

impulse response we can easily predict the signal obtained at the receiver from the transmitter.

EFFECT OF PROPAGATION OF MOBILE SIGNALS OVER WATER AND FLAT OPEN AREA

PROPAGATION OVER WATER OR FLAT OPEN AREA:

Propagation over water or fiat open area is becoming a big concern because it is very easy to interfere with

other cells if we do not make the correct arrangements. Interference resulting from propagation over the water

can be controlled if we know the cause. In general, the permittivity’s Er of seawater and fresh water are the same,

but the conductivities of seawater and fresh water are different. We may calculate the dielectric constants Ec

where Ec = Er - j60σλ. The wavelength at 850MHz is 0.35m. Then Eo (sea water) = 80 - j84 and Ec (fresh water)=80-

jO.021.

However, based upon the reflection coefficients formula with a small incident angle both the reflection

coefficients for horizontal polarized waves and vertically polarized waves approach 1. Since the 180* phase change

occurs at the ground reflection point, the reflection coefficient is -1. Now we can establish a scenario, as shown in

Fig 10.1 Since the two antennas, one at the cell site and the other at the mobile unit, are well above sea level, two

reflection points are generated. The one reflected from the ground is close to the mobile unit; the other reflected

from the water is away from the mobile unit. We recall that the only reflected wave we considered in the land

mobile propagation is the one reflection point which is always very close to the mobile unit. We are now using the

formula to find the field strength under the circumstances of a fixed point-to-point transmission and a land-mobile

transmission over a water or flat open land condition.


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Fig 10.1.A model for propagation over water

BETWEEN FIXED STATIONS: The point -to-point transmission between the fixed stations over the water or

flat open land can be estimated as follows. The received power P, can be expressed as (see Fig.10.2)

Fig 10.2.Propagation between two fixed stations over water or flat open land.

_φ is the phase difference caused by the path difference M between the direct wave and the reflected wave, or

The first part of i.e. the free-space loss formula which shows the 20 dB/dec slope; that is, a 20-dB loss will be seen

when propagating from 1 to 10 km.


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The complex reflection co-efficient and can be found from the formula

When the vertical incidence is small, θ is very small and

It can be found from equation. Ec is a dielectric constant that is different for different media. The reflection

coefficient remains -1 regardless of whether the wave is propagated over water dry land, wet land, Ice, and so

forth. The wave propagating between fixed stations is illustrated in Fig. 10.2.

since _φ is a function of d and d can be obtained from the following calculation. The effective antenna height at

antenna 1 is the height above the sea level.

The effective antenna height at antenna 2 is the height above the sea level.

As shown in Fig.10.2 where h1 and h2 are actual heights and H1and H2 are the heights of hills. In general,

both antennas at fixed stations are high, so the resection point of the wave will be found toward the middle of the

radio path. The path difference d can be obtained from Fig. 10.2 as

Then


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MOBILE-TO-MOBILE PROPAGATION In mobile-to-mobile land communication, both the transmitter and the receiver are in motion. The

propagation path in this case is usually obstructed by buildings and obstacles between the transmitter and

receiver. The propagation channel acts like a filter with a time-varying transfer function H (f, t) which can be found

in this section.

The two mobile units M1 and M2 with velocities V1 and V2 respectively are shown in Fig.11.1. Assume that the

transmitted signal from M1 is

Where a1i and a2i are random angles as shown in Fig.11.1. Now assume that the received signal is the summation

of n paths uniformly distributed around the azimuth.


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UNIT IV (A)

CELLSITE AND MOBILE ANTENNAS

SPACES-DIVERSITY ANTENNAS

Two-branch space-diversity antennas are used at the cell site to receive the same signal with different

fading envelopes, one at each antenna. The degree of correlation between two fading envelopes is determined by

the degree of separation between two receiving antennas. When the two fading envelopes are combined, the

degree of fading is reduced. Here the antenna setup is shown in Fig. 5a.

Equation is presented as an example for the designer to use.

η = h/D = 11 (8.13-1)

Where h is the antenna height and D is the antenna separation. From Eq., the separation d ≥ 8λ is

needed for an antenna height of 100 ft (30 m) and the separation d ≥ 14λ is needed for an antenna height of 150

ft (50 m). In any Omni cell system, the two space-diversity antennas should be aligned with the terrain, which

should have a U shape as shown in Fig.5b. Space-diversity antennas can separate only horizontally, not vertically;

thus, there is no advantage in using a vertical separation in the design.

Fig.6.10.Diversity antenna spacing at cell site: (a) n=h/d (b) Proper

arrangement with two antennas

UMBRELLAS-PATTERN ANTENNAS

In certain situations, umbrella-pattern antennas should be used for the cell-site antennas.

Fig. Vertical-plane patterns of quarter-wavelength stub antenna on infinite ground plane

(solid) and on finite ground planes several wavelengths in diameter (dashed line) and

about one wavelength in diameter (dotted line).


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i) NORMAL UMBRELLA-PATTERN ANTENNA:

For controlling the energy in a confined area, the umbrella-pattern antenna can be developed by using a

monopole with a top disk (top-loading) as shown in Fig. The size of the disk determines the tilting angle of the

pattern. The smaller the disk, the larger the tilting angle of the umbrella pattern.

ii) BROADBAND UMBRELLA-PATTERN ANTENNA:

The parameters of a Discone antenna (a bio conical antenna in which one of the cones is extended to

180◦ to form a disk) are shown in Fig. The diameter of the disk, the length of the cone, and the opening of the

cone can be adjusted to create an umbrella-pattern antenna.

iii) INTERFERENCE REDUCTION ANTENNA:

A design for an antenna configuration that reduces interference in two critical directions (areas) is shown

in Fig.6.3. The parasitic (insulation) element is about 1.05 times longer than the active element.

iv) HIGH-GAIN BROADBAND UMBRELLA-PATTERN ANTENNA:

A high-gain antenna can be constructed by vertically stacking a number of umbrella-

pattern antennas as shown in Fig.

Fig. Discone antennas (a) Single antenna; (b) An array of antenna

MINIMUM SEPARATION OF CELL-SITE RECEIVING ANTENNAS

Separation between two transmitting antennas should be minimized to avoid the inter modulation. The

minimum separation between a transmitting antenna and a receiving antenna is necessary to avoid receiver


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desensitization. Here we are describing a minimum separation between two receiving antennas to reduce the

antenna pattern ripple effects. The two receiving antennas are used for a space-diversity receiver.

Because of the near field disturbance due to the close spacing, ripples will form in the antenna patterns

(Fig.). The difference in power reception between two antennas at different angles of arrival is shown in Fig. . If

the antennas are located closer; the difference in power between two antennas at a given pointing angle

increases. Although the power difference is confined to a small sector, it affects a large section of the street as

shown in Fig. .

If the power difference is excessive, use of space diversity will have no effect reducing fading. At 850 MHz, the

separation of eight wavelengths between two receiving antennas creates a power difference of ±2 dB, which is

tolerable for the advantageous use of a diversity scheme.

Fig. Antenna pattern ripple effect

MOBILE ANTENNAS The requirement of a mobile (motor-vehicle–mounted) antenna is an Omni-directional antenna that can

be located as high as possible from the point of reception. However, the physical limitation of antenna height on

the vehicle restricts this requirement. Generally, the antenna should at least clear the top of the vehicle. Patterns

for two types of mobile antenna are shown in Fig.


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Fig. Mobile antenna patterns (a) Roof mounted 3-dB-gain collinear antenna

versus roof-mounted quarter-wave antenna, (b) Window-mourned “on-glass”

gain antenna versus roof-mounted quarter-wave antenna.

ROOF-MOUNTED ANTENNA:

The antenna pattern of a roof-mounted antenna is more or less uniformly distributed around the mobile

unit when measured at an antenna range in free space as shown in Fig.9.2. The 3-dBhigh-gain antenna shows a 3-

dBgain over the quarter-wave antenna. However, the gain of the antenna used at the mobile unit must be limited

to 3 dB because the cell-site antenna is rarely as high as the broadcasting antenna and out-of-sight conditions

often prevail. The mobile antenna with a gain of more than 3 dB can receive only a limited portion of the total

multipath signal in the elevation as measured under the out-of-sight condition.

Fig. Vertical angle of signal arrival

GLASS-MOUNTED ANTENNAS:

There are many kinds of glass-mounted antennas. Energy is coupled through the glass; therefore,

there is no need to drill a hole. However, some energy is dissipated on passage through the glass. The antenna

gain range is 1 to 3 dB depending on the operating frequency. The position of the glass-mounted antenna is


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always lower than that of the roof-mounted antenna; generally there is a 3-dBdifference between these two

types of antenna. Also, glass mounted antennas cannot be installed on the shaded glass found in some motor

vehicles because this type of glass has a high metal content.

MOBILE HIGH-GAIN ANTENNAS:

A high-gain antenna used on a mobile unit has been studied. This type of high-gain antenna should be

distinguished from the directional antenna. In the directional antenna, the antenna beam pattern is suppressed

horizontally; in the high-gain antenna, the pattern is suppressed vertically.

To apply either a directional antenna or a high-gain antenna for reception in a radio environment, we

must know the origin of the signal. If we point the directional antenna opposite to the transmitter site, we

would in theory receive nothing. In a mobile radio environment, the scattered signals arrive at the mobile unit

from every direction with equal probability. That is why an Omni directional antenna must be used.

The scattered signals also arrive from different elevation angles. Lee and Brandt used two types of

antenna, one λ/4 whip antenna with elevation coverage of 39◦ and one 4-dB-gain antenna (4-dB gain with respect

to the gain of a dipole) with elevation coverage of 16◦ and measured the angle of signal arrival in the suburban

Keyport-Matawan area of New Jersey. There are two types of test: a line-of-sight condition and an out-of-sight

condition. In Lee and Brandt’s study, the transmitter was located at an elevation of approximately 100 m (300 ft)

above sea level.

The measured areas were about 12 m (40 ft) above sea level and the path length about 3 mi. The

received signal from the 4-dB-gain antenna was 4 dB stronger than that from the whip antenna under line-of-

sight conditions. This is what we would expect.

However, the received signal from the 4-dB-gain antenna was only about 2 dB stronger than that from

the whip antenna under out-of-sight conditions. This is surprising. The reason for the latter observation is that the

scattered signals arriving under out-of- sight conditions are spread over a wide elevation angle. A large portion of

the signals outside the elevation angle of 16◦ cannot be received by the high-gain antenna. We may calculate the

portion being received by the high-gain antenna from the measured beam width. For instance, suppose that a 4:1

gain (6 dBi) is expected from the high-gain antenna, but only 2.5:1 is received. Therefore, 63 percent of the signal

is received by the 4-dB-gain antenna (i.e., 6 dBi) and 37 percent is felt in the region between 16 and 39◦

Therefore, a 2- to 3-dB-gain antenna (4 to 5 dBi) should be adequate for general use. An antenna gain

higher than 2 to 3 dB does not serve the purpose of enhancing reception level. Moreover, measurements reveal

that the elevation angle for scattered signals received in urban areas is greater than that in suburban areas.


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UNIT-IV (B)

FREQUENCY MANAGEMENT AND CHANNEL ASSIGNMENT

The function of frequency management is to divide the total number of available channels into subsets

which can be assigned to each cell either in a fixed fashion or dynamically (i.e., in response to any channel among

the available channels). The terms ―frequency management‖ and ―channel assignment‖ often create some

confusion. Frequency management refers to designating setup channels and voice channels (done by the FCC),

numbering the channels (done by the FCC), and grouping the voice channels into subsets (done by each system

according to its preference).

Channel assignment refers to the allocation of specific channels to cell sites and mobile units. A fixed

channel set consisting of one more subsets is assigned to a cell site on a long-term basis. During a call, a particular

channel is assigned to a mobile unit on a short- term basis. For a short-term assignment, one channel assignment

per call is handled by the mobile telephone switching office (MTSO). Ideally channel assignment should be based

on causing the least interference in the system. However, most cellular systems cannot perform this way.

4.1 NUMBERING THE RADIO CHANNELS

The total number of channels at present (January 1988) is 832. But most mobile units an systems are still

operating on 666 channels. Therefore we describe the 666 channel numbering first. A channel consists of two

frequency channel bandwidths, one in the low band and one in the high band. Two frequencies in channel 1 are

825.030 MHz (mobile transmit) 870.030 MHz (cell-site transmit). The two frequencies in channel 666 are 844.98

MHz (mobile transmit) and 898 MHz (cell-site transmit). The 666 channels are divided into two groups: block A

system and block B system. Each market (i.e., each city) has two systems for a duopoly market policy. Each block

has 333 channels, as shown in Fig.1.1.

The 42 set-up channels are assigned as follows.

Channels 313-333 block A

Channels 334-354 block B

The voice channels are assigned as follows.

Channels 1-312 (312 voice channels)

block

A

block

Channels 355-666 (312 voice channels) B


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Fig.4.1. Frequency management chart

These 42 set-up channels are assigned in the middle of all the assigned channels to facilitate scanning of

those channels by frequency synthesizers. In the new additional spectrum allocation of 10 MHz (sec Fig. 1.2.), an

additional 166 channels are assigned. Since a 1 MHz is assigned below 825 MHz (or 870 MHz) in the future,

additional channels will be numbered up to 849 MHz (or 894 MHz) and will then circle back. The last channel

number is 1023. There are no Channels between channels 799 and 991.

Fig.4.2. New additional spectrum allocation


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4.2 GROUPING INTO SUBSETS

The number of voice channels for each system is 312. We can group these into any number of subsets.

Since there are 21 set-up channels for each system, it is logical to group the 312 channels into 21 subsets. Each

subset then consists of 16 channels. In each set, the closest adjacent channel is 21 channels away, as shown in

Fig.1.1. The 16 channels in each subset can be mounted on a frame and connected to a channel combiner. Wide

separation between adjacent channels is required for meeting the requirement of minimum isolation. Each 16-

channel subset is idealized for each 16-channel combiner. In a seven- cell frequency-reuse cell system each cell

contains three subsets, iA+iB+iC, where i is an integer from 1 to 7. The total number of voice channels in a cell is

about 45. The minimum separation between three subsets is 7 channels. If six subsets are equipped in an

omnicell site, the minimum separation between two adjacent channels can be only three (21/6> 3) physical

channel bandwidths.

For example,

1A+1B+1C+4A+4B

+4 C

Or

1A+1B+1C+5A+5B+5C

4.3 SET-UP CHANNELS

Set-up channels also called control channels are the channels designated to setup calls. We should not be

confused by fact that a call always needs a set-up channel. A system can be operated without set-up channels. If

we are choosing such a system all the 333 channels in each cellular system (block A or block B) can be voice

channels; however each mobile unit must then scan 333 channels continuously and detect the signaling for its

call. A customer who wants to initiate a call must scan all the channels and find an idle (unoccupied) one to use.

In a cellular system, we are implementing frequency-reuse concepts. In this case the set-up channels are

acting as control channels. The 21 set-up channels are taken out from the total number of channels. The number

21 is derived from a seven-cell frequency-reuse pattern with three 120◦ sectors per cell, or a total of 21 sectors,

which require 21 set-up channels. However, now only a few of the 21 setup channels are being used in each

system. Theoretically, when cell size decreases the use of set-up channels should increase. Set-up

channels can be classified by usage into two types: access channels and paging channels.

An access channel is used for the mobile-originating calls and paging channels for the land originating

calls. For this reason, a set-up channel is sometimes called an ‗access channel‘ and sometimes called a ‗paging

channel.‘ Every two- way channel contains two 30-kHz bandwidth.. Normally one set-up channel is also specified


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by two operations as a forward set-up channel (using the upper band) and a reverse set-up channel (using the

lower band). In the most common types of cellular systems, one set-up channel is used for both access and

paging. The forward set-up channel functions as the paging channel for responding to the mobile-originating calls.

The reverse set-up channel functions as the access channel for the responder to the paging call. The forward set-

up channel is transmitted at the cell site, and the reverse set-up channel is transmitted at the mobile unit. All set-

up channels carry data information only.

4.3.1. ACCESS CHANNELS:

In mobile-originating calls, the mobile unit scans its 21 set-up channels and chooses the strongest one.

Because each set-up channel is associated with one cell, the strongest set-up channel indicates which cell is to

serve the mobile-originating calls. Th. mobile unit detects the system information transmitted from the cell site.

Also, the mobile unit monitors the Busy/Idle status bits over the desired forward setup channel. When the idle

bits are received, the mobile unit can use the corresponding reverse set-up channel to initiate a call.

Frequently only one system operates in a given city; for instance, block B system might be operating and

the mobile unit could be set to ―preferable A system.‖ When the mobile unit first scans the 21 set-up channels in

block A, two conditions can occur.

1. If no set-up channels of block A are operational, the mobile unit automatically switches to block B.

2. If a strong set-up signal strength is received but no message can be detected, then the scanner chooses the

second strongest set-up channel. If the message still cannot be detected, the mobile unit switches to block B and

scans to block B set-up channels.

THE OPERATIONAL FUNCTIONS ARE DESCRIBED AS FOLLOWS:

1.POWER OF A FORWARD SET-UP CHANNEL [OR FORWARD CONTROL CHANNEL (FOCC)]: The power of the set-

up channel can be varied in order to control the number of incoming calls served by the cell. The number of

mobile-originating calls is limited by the number of voice channels in each cell site, when the traffic is heavy, most

voice channels are occupied and the power of the set-up channel should be reduced in order to reduce the

coverage of the cell for the incoming calls originating from the mobile unit. This will force the mobile units to

originate calls from other cell sites, assuming that all cells are adequately overlapped.

2. THE SET-UP CHANNEL RECEIVED LEVEL: The setup channel threshold level is determined in order to control the

reception at the reverse control channel (RECC). If the received power level is greater than the given set-up

threshold level, the call request will be taken.


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3. CHANGE POWER AT THE MOBILE UNIT: When the mobile unit monitors the strongest signal strength from all

Set-up channels and selects that channel to receive the messages, there are three types of message.

A. MOBILE STATION CONTROL MESSAGE. This message is used for paging and consists of one, two, or

four words -DCC, MIN, SCC and VMAX.

B. SYSTEM PARAMETER OVERHEAD MESSAGE. This message contains two words, including DCC, SID,

CMAX, or CPA.

c. CONTROL-FILLER MESSAGE. This message may be sent with a system parameter overhead message,

CMAC—a control mobile attenuation code (seven levels).

4. DIRECT CALLS RETRY. When a cell site has no available voice channels, it can send a direct call-retry message

through the set-up channel. The mobile unit will initiate, the call from a neighboring cell which is on the list of

neighboring cells in the direct call-retry message.

4.3.2. PAGING CHANNELS:

Each cell site has been allocated its own setup channel (control channel). The assigned forward set-up

channel (FOCC) of each cell site is used to page the mobile unit with the same mobile station control message.

Because the same message is transmitted by the different set-up channels, no simulcast interference

occurs in the system. The algorithm for paging & mobile unit can be performed in different ways. The simplest

way is to page from all the cell sites. This can occupy a large amount of the traffic load. The other way is to page

in an area corresponding to the mobile unit phone number. If there is no answer, the system tries to page in

other areas. The drawback is that response time is sometimes too long.

When the mobile unit responds to the page on the reverse set-up channel, the cell site which receives

the response checks the signal reception level and makes a decision regarding the voice channel assignment

based on least interference in the selected sector or underlay-overlay region.

4.4 FIXED CHANNEL ASSIGNMENT

ADJACENT-CHANNEL ASSIGNMENT: Adjacent-channel assignment includes neighboring-channel assignment and next-channel assignment. The

near-end–far-end (ratio) interference can occur among the neighboring channels (four channels on each side of

the desired channel). Therefore, within a cell we have to be sure to assign neighboring channels in an Omni-

directional-cell system and in a directional-antenna-cell system properly.

In an Omni-directional-cell system, if one channel is assigned to the middle cell of seven cells, next

channels cannot be assigned in the same cell. Also, no next channel (preferably including neighboring channels)

should be assigned in the six neighboring sites in the same cell system area (Fig. 7.3a). In a directional-antenna-


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cell system, if one channel is assigned to a face, next channels cannot be assigned to the same face or to the other

two faces in the same cell. Also, next channels cannot be assigned to the other two faces at the same cell site (Fig.

7.3b). Sometimes the next channels are assigned in the next sector of the same cell in order to increase capacity.

Then performance can still be in the tolerance range if the design is proper.

Fig.4.3 Adjacent channel assignment (a) Omni direction antenna

cells; (b) Directional antenna cells

4.5 CHANNEL SHARING

Channel sharing is a short-term traffic-relief scheme. A scheme used for a seven-cell three-face system is

shown in Fig. 7.2. There are 21 channel sets, with each set consisting of about 16 channels. Figure7.2 shows the

channel set numbers. When a cell needs more channels, the channels of another face at the same cell site can be

shared to handle the short-term overload. To obey the adjacent-channel assignment algorithm, the sharing is

always cyclic. Sharing always increases the trunking efficiency of channels.

Since we cannot allow adjacent channels to share with the nominal channels in the same cell, channel

sets 4 and 5 cannot both be shared with channel sets 12 and 18, a indicated by the grid mark. Many grid marks

are indicated in Fig.7.2 for the same reason. However, the upper subset of set 4 can be shared with the lower

subset of set 5 with no interference. In channel-sharing systems, the channel combiner should be flexible in order

to combine up to 32 channels in one face in real time. An alternative method is to install a standby antenna.


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Fig.4.4. Channel sharing algorithm

4.6 CHANNEL BORROWING

Channel borrowing is usually handled on a long-term basis. The extent of borrowing more available

channels from other cells depends on the traffic density in the area. Channel borrowing can be implemented from

one cell-site face to another face at the same cell site. In addition, the central cell site can borrow channels from

neighboring cells. The channel-borrowing scheme is used primarily for slowly-growing systems. It is often helpful

in delaying cell splitting in peak traffic areas. Since cell splitting is costly, it should be implemented only as a last

resort.

ADVANTAGE OF SECTORIZATION:

The total number of available channels can be divided into sets (subgroups) depending on the

Sectorization of the cell configuration: the 120◦-sector system, the 60◦-sector system, and the 45◦-sector system.

In certain locations and special situations, the sector angle can be reduced (narrowed) in order to assign more

channels in one sector without increasing neighboring-channel interference. Sectorization serves the same

purpose as the channel-borrowing scheme in delaying cell splitting. In addition, channel coordination to avoid co-

channel interference is much easier in sectorization than in cell splitting. Given the same number of channels,

trunking efficiency decreases in Sectorization.

SECTORIZED CELLS: There are three basic types.

1. The 120◦-sector cell is used for both transmitting and receiving Sectorization. Each sector has an assigned a

number of frequencies. Changing sectors during a call requires handoffs.

2. The 60◦-sector cell is used for both transmitting and receiving Sectorization. Changing sectors during a call

requires handoffs. More handoffs are expected for a 60◦ sector than a 120◦ sector in areas close to cell sites

(close-in areas).


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3. The 120◦ or 60◦-sector cell is used for receiving Sectorization only. In this case, the transmitting antenna is

Omni directional. The number of channels in this cell is not sub- divided for each sector. Therefore, no handoffs

are required when changing sectors. This receiving-Sectorization-only configuration does not decrease

interference or increase the D/R ratio; it only allows for a more accurate decision regarding handing off the calls

to neighboring cells.

4.7 UNDERLAY-OVERLAY ARRANGEMENT

In actual cellular systems cell grids are seldom uniform because of varying traffic conditions in different areas and

cell-site locations.

OVERLAID CELLS:

To permit the two groups to reuse the channels in two different cell-reuse patterns of the same size, an

―under laid‖ small cell is sometimes established at the same cell site as the large cell (see Fig. 7.5a). The

―doughnut‖ (large) and ―hole‖ (small) cells are treated as two different cells. They are usually considered as

―neighboring cells.‖

Fig.4.5.Under laid-overlaid cell arrangements. (a) Underlay-overlay in omnicell; (b) Underlay-

overlay in Sectorized cell; (c) Two level handoff scheme

The use of either an Omni directional antenna at one site to create two sub ring areas or three

directional antennas to create six subareas is illustrated in Fig. 4.5 b. As seen in Fig.4.5, a set of frequencies used

in an overlay area will differ from a set of frequencies used in an underlay area in order to avoid adjacent-channel

and co-channel interference.

The channels assigned to one combiner—say, 16 channels—can be used for overlay, and another combiner can

be used for underlay.

IMPLEMENTATION:

The antenna of a set-up channel is usually Omni directional. When an incoming call is received by the set-up

channel and its signal strength is higher than a level L, the under laid cell is assigned; otherwise, the overlaid cell is


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assigned. The handoffs are implemented between the under laid and overlaid cells. In order to avoid the

unnecessary handoffs, we may choose two levels L1 and L2 and L1 > L2 as shown in Fig. 4.5 (c). When a mobile

signal is higher than a level L1 the call is handed off to the under laid cell. When a signal is lower than a level L2

the call is handed off to the overlaid cell. The channels assigned in the under laid cell have more protection

against co-channel interference.

4.8 NON FIXED CHANNEL ASSIGNMENT STRATEGY

1. FIXED CHANNEL ASSIGNMENT: The fixed channel assignment (FCA) algorithm is the most common algorithm

adopted in many cellular systems. In this algorithm, each cell assigns its own radio channels to the vehicles within

its cell.

2. DYNAMIC CHANNEL ASSIGNMENT: In dynamic channel assignment (DCA), no fixed channels are assigned to

each cell. Therefore, any channel in a composite of N radio channels can be assigned to the mobile unit. This

means that a channel is assigned directly to a mobile unit. On the basis of overall system performance, DCA can

also be used during a call.

3. HYBRID CHANNEL ASSIGNMENT: Hybrid channel assignment (HCA) is a combination of FCA and DCA. A portion

of the total frequency channels will use FCA and the rest will use DCA.

4. BORROWING CHANNEL ASSIGNMENT: Borrowing channel assignment (BCA) uses FCA as a normal assignment

condition. When all the fixed channels are occupied, then the cell borrows channels from the neighboring cells.

5. FORCIBLE-BORROWING CHANNEL ASSIGNMENT: In forcible-borrowing channel assignment (FBCA), if a

channel is in operation and the situation warrants it, channels must be borrowed from the neighboring cells and

at the same time, another voice channel will be assigned to continue the call in the neighboring cell. There are

many different ways of implementing FBCA. In a general sense, FBCA can also be applied while accounting for the

forcible borrowing of the channels within a fixed channel set to reduce the chance of co-channel assignment in a

reuse cell pattern. The FBCA algorithms based on assigning a channel dynamically but obeying the rule of reuse

distance.

The distance between the two cells is reuse distance, which is the minimum distance at which no co-

channel interference would occur. Very infrequently, no channel can be borrowed in the neighboring cells. Even

those channels currently in operation can be forcibly borrowed and will be replaced by a new channel in the

neighboring cell or the neighboring cell of the neighboring cell. If all the channels in the neighboring cells cannot

be borrowed because of interference problems, the FBCA stops.


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UNIT-V

HANDOFFS

WHY HAND OFF IS NECESSARY

In an analog system, once a call is established, the set-up channel is not used again during the call

period. Therefore, handoff is always implemented on the voice channel. In the digital systems, the handoff is

carried out through paging or common control channel. The value of implementing handoffs is dependent on

the size of the cell. For example, if the radius of the cell is 32 km (20 mi), the area is 3217 km^2(1256

mi^2). After a call is initiated in this area, there is little chance that it will be dropped before the call is terminated

as a result of a weak signal at the coverage boundary. Then why bother to implement the handoff feature? Even

for a 16-km radius, cell handoff may not be needed. If a call is dropped in a fringe area, the customer simply

redials and reconnects the call. Today the size of cells becomes smaller in order to increase capacity. Also people

talk longer. The handoffs are very essential. Handoff is needed in two situations where the cell site receives

weak signals from the mobile unit: (1) at the cell boundary, say, −100 dBm, which is the level for requesting a

handoff in a noise-limited environment; and (2) when the mobile unit is reaching the signal-strength holes (gaps)

within the cell site as shown in Fig.1.

Fig.1. Occurrence of handoffs

WHAT ARE THE TWO DECISIONS MAKING PARAMETERS OF HANDOFF EXPLAIN

There are two decision-making parameters of handoff: (1) that based on signal strength and (2) that

based on carrier-to-interference ratio. The handoff criteria are different for these two types. In type 1, the


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signal-strength threshold level for handoff is −100 dBm in noise-limited systems and −95 dBm in interference-

limited systems. In type 2, the value of C/I at the cell boundary for handoff should be at a level, 18 dB for AMPS

in order to have toll quality voice. Sometimes, a low value of C/I may be used for capacity reasons.

Type 1: It is easy to implement. The location receiver at each cell site measures all the signal strengths of all

receivers at the cell site. However, the received signal strength (RSS) itself includes interference.

RSS = C + I

where C is the carrier signal power and I is the interference. Suppose that we set up a threshold level for RSS; then,

because of the I , which is sometimes very strong, the RSS level is higher and far above the handoff threshold level.

In this situation handoff should theoretically take place but does not. Another situation is when I is very low but

RSS is also low. In this situation, the voice quality usually is good even though the RSS level is low, but since RSS is

low, unnecessary handoff takes place. Therefore, it is an easy but not very accurate method of determining

handoffs. Some analog systems use SAT information together with the received signal level to determine

handoffs. Some CDMA systems use pilot channel information.

Type 2: Handoffs can be controlled by using the carrier-to-interference ratio C/I C+I/I = C/I

we can set a level based on C/I ,so C drops as a function of distance but I is dependent on the location. If the

handoff is dependent on C/I , and if the C/I drops, it does so in response to increase in (1) propagation distance or

(2) interference. In both cases, handoff should take place. In today’s cellular systems, it is hard to measure

C/I during a call because of analog modulation. Sometimes we measure the level I before the call is

connected, and the level C + I during the call. Thus (C + I )/I can be obtained.

TYPES OF HANDOFF

There are four types of handoff:

1. INTERSECTOR OR SOFTER HANDOFF.

The mobile communicates with two sectors of the same cell (see Fig. 10-1). A RAKE receiver at the base

station combines the best versions of the voice frame from the diversity antennas of the two sectors into a

single traffic frame.

2. INTERCELL OR SOFT HANDOFF.

The mobile communicates with two or three sectors of different cells (see Fig. 10-2). The base station that

has the direct control of call processing during handoff is referred to as the primary base station. The

primary base station can initiate the forward control message. Other base stations that do not have control

over call processing are called the secondary base stations. Soft handoff ends when either the primary or

secondary base station is dropped. If the primary base station is dropped, the secondary base station

becomes the new primary for this call. A three-way soft handoff may end by first dropping one of the base

stations and becoming a two-way soft handoff. The base stations involved coordinate handoff by


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exchanging information via SS7 links. A soft handoff uses considerably more network resources than the

softer handoff.

3. SOFT-SOFTER HANDOFF.

The mobile communicates with two sectors of one cell and one sector of another cell (see Fig. 10-3).

Network resources required for this type of handoff include the resources for a two-way soft handoff

between cell A and B plus the resources for a softer handoff at cell B.

4. HARD HANDOFF.

Hard handoffs are characterized by the break-before-make strategy. The connection with the old traffic

channel is broken before the connection with the new traffic channel is established. Scenarios for hard

handoff include

◆ Handoff between base stations or sectors with different CDMA carriers

◆ Change from one pilot to another pilot without first being in soft handoff with the new pilot (disjoint

active sets)

◆ Handoff from CDMA to analog, and analog to CDMA

◆ Change of frame offset assignment—CDMA traffic frames are 20 ms long. The start of frames in a

particular traffic channel can be at 0 time in reference to a system or it can be offset by up to 20 ms (allowed in IS-

95). This is known as the frame offset. CDMA traffic channels are assigned different frame offset to avoid

congestion. The frame offset for a particular traffic channel is communicated to the mobile. Both forward and

reverse links use this offset. A change in offset


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Assignment will disrupt the link. During soft handoff the new base station must allocate the same frame offset to

the mobile as assigned by the primary base station. If that particular frame offset is not available, a hard handoff

may be required. Frame offset is a network resource and can be used up

HANDOFF INITIATION

A hard handoff occurs when the old connection is broken before a new connection is activated. The

performance evaluation of a hard handoff is based on various initiation criteria [1, 3, 13]. It is assumed that the

signal is averaged over time, so that rapid fluctuations due to the multipath nature of the radio environment can

be eliminated. Numerous studies have been done to determine the shape as well as the length of the averaging

window and the older measurements may be unreliable. Figure 1.2 shows a MS moving from one BS (BS1) to

another (BS2). The mean signal strength of BS1 decreases as the MS moves away from it. Similarly, the mean signal

strength of BS2 increases as the MS approaches it. This figure is used to explain various approaches described in

the following subsection.

1.3.1 RELATIVE SIGNAL STRENGTH

This method selects the strongest received BS at all times. The decision is based on a mean measurement of

the received signal. In Figure 1.2, the handoff would occur at position A. This method is observed to provoke too

many unnecessary handoffs, even when the signal of the current BS is still at an acceptable level.

1.3.2 RELATIVE SIGNAL STRENGTH WITH THRESHOLD

This method allows a MS to hand off only if the current signal is sufficiently weak (less than threshold) and the

other is the stronger of the two. The effect of the threshold depends on its relative value as compared to the signal

strengths of the two BSs at the point at which they are equal. If the threshold is higher than this value, say T1 in

Figure 1.2, this scheme performs exactly like the relative signal strength scheme, so the handoff occurs at position

A. If the threshold is lower than this value, say T2 in Figure 1.2, the MS would delay handoff until the current signal

level crosses the threshold at position B. In the case of T3, the delay may be so long that the MS drifts too far into

the new cell.


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This reduces the quality of the communication link from BS1 and may result in a dropped call. In addition, this

results in additional interference to cochannel users. Thus, this scheme may create overlapping cell coverage

areas. A threshold is not used alone in actual practice because its effectiveness depends on prior knowledge of the

crossover signal strength between the current and candidate BSs.

1.3.3 RELATIVE SIGNAL STRENGTH WITH HYSTERESIS

This scheme allows a user to hand off only if the new BS is sufficiently stronger (by a hysteresis margin, h in

Figure 1.2) than the current one. In this case, the handoff would occur at point C. This technique prevents the so-

called ping-pong effect, the repeated handoff between two BSs caused by rapid fluctuations in the received signal

strengths from both BSs. The first handoff, however, may be unnecessary if the serving BS is sufficiently strong.

1.3.4 RELATIVE SIGNAL STRENGTH WITH HYSTERESIS AND THRESHOLD

This scheme hands a MS over to a new BS only if the current signal level drops below a threshold and the

target BS is stronger than the current one by a given hysteresis margin. In Figure 1.2, the handoff would occur at

point D if the threshold is T3

1.3.5 PREDICTION TECHNIQUES

Prediction techniques base the handoff decision on the expected future value of the received signal strength.

A technique has been proposed and simulated to indicate better results, in terms of reduction in the number of

unnecessary handoffs, than the relative signal strength, both without and with hysteresis, and threshold methods.

CONCEPT OF DELAYING A HANDOFF

In many cases, a two-handoff-level algorithm is used. The purpose of creating two request handoff

levels is to provide more opportunity for a successful handoff. A handoff could be delayed if no available cell

could take the call. A plot of signal strength with two request handoff levels and a threshold level is shown

in Fig.3. The plot of average signal strength is recorded on the channel received


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Fig.3. A two level handoff scheme

Signal strength indicator (RSSI), which is installed at each channel receiver at the cell site. When the signal

strength drops below the first handoff level, a handoff request is initiated. If for some reason the mobile unit is in a

hole (a weak spot in a cell) or a neighboring cell is busy, the handoff will be requested periodically every 5 s. At the

first handoff level, the handoff takes place if the new signal is stronger. However, when the second handoff level is

reached, the call will be handed off with no condition. The MSO always handles the handoff call first and the

originating calls second. If no neighboring calls are available after the second handoff level is reached, the call

continues until the signal strength drops below the threshold level; then the call is dropped. In AMPS

systems if the supervisory audio tone (SAT) is not sent back to the cell site by the mobile unit within 5 s, the cell

site turns off the transmitter.

ADVANTAGES OF DELAYED HANDOFF

1. Consider the following example. The mobile units are moving randomly and the terrain contour is

uneven. The received signal strength at the mobile unit fluctuates up and down. If the mobile unit is

in a hole for less than 5 s (a driven distance of 140 m for 5 s, assuming a vehicle speed of 100 km/h),

the delay (in handoff) can even circumvent the need for a handoff. If the neighboring cells are busy,

delayed handoff may take place. In principle, when call traffic is heavy, the switching processor is

loaded, and thus a lower number of handoffs would help the processor handle call processing more

adequately. Of course, it is very likely that after the second handoff level is reached, the call may be

dropped with great probability.

2. The other advantage of having a two-handoff-level algorithm is that it makes thehandoff occur at

the proper location and eliminates possible interference in the system. Figure 3, case I, shows the

area where the first-level handoff occurs between cell A and cell B. If we only use the second-level

handoff boundary of cell A, the area of handoff is too close to cell B. Figure 3, case II, also shows

where the second-level handoff occurs between cell A and cell C. This is because the first-level

handoff cannot be implemented.

POWER DIFFERENCE HANDOFF

A better algorithm is based on the power difference (_) of a mobile signal received by two cell sites, home and

handoff. _ can be positive or negative. The handoff occurs depending on a preset value of _.


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_ = the mobile signal measured at the candidate handoff site

− The mobile signal measured at the home site

For example, the following cases can occur.

_> 3 dB request a handoff

1dB <_< 3 dB prepare a handoff

−3dB <_< 0 dB monitoring the signal strength

_< −3 dB no handoff

Those numbers can be changed to fit the switch processor capacity. This algorithm is not based on

the received signal strength level, but on a relative (power difference) measurement. Therefore, when this

algorithm is used, all the call handoffs for different vehicles can occur at the same general location in spite of

different mobile antenna gains or heights.

FORCED HANDOFF

A forced handoff is defined as a handoff that would normally occur but is prevented from happening, or a handoff

that should not occur but is forced to happen.

MOBILE-ASSISTED HANDOFF

In a mobile-assisted handoff process, the MS makes measurements and the network makes thedecision. In the

circuit switched GSM (global system mobile), the BS controller (BSC) is in charge of the radio interface

management. This mainly means allocation and release of radio channels and handoff management. The handoff

time between handoff decision and execution in such a circuit-switched GSM is approximately 1 second.

SOFT HANDOFF

SOFT HANDOFF (FORWARD LINK)

In this case all traffic channels assigned to the mobile are associated with pilots in the active set and carry the

same traffic information with the exception of power control subchannel. When the active set contains more than

one pilot, the mobile provides diversity by combining its associated forward traffic channels.

SOFT HANDOFF (REVERSE LINK)

During intercell handoff, the mobile sends the same information to both base stations. Each base station

receives the signal from the mobile with appropriate propagation delay. Each base station then transmits the

received signal to the vocoder/selector. In other words, two copies of the same frame are sent to the

vocoder/selector. The vocoder/selector selects the better frame and discards the other.

SOFTER HANDOFF (REVERSE LINK)

During intersector handoff, the mobile sends the same information to both sectors. The channel

card/element at the cell site receives the signals from both sectors. The channel card combines both inputs, and

only one frame is sent to the vocoder/selector. It should be noted that extra channel cards are not required to

support softer handoff as is the case for soft handoffs. The diversity gain from soft handoffs is more than the


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diversity gain from softer handoffs because signals from distinct cells are less correlated than signals from sectors

of the same cell.

10.2.4 BENEFIT OF SOFT HANDOFF

A key benefit of soft handoff is the path diversity on the forward and reverse traffic channels. Diversity gain is

obtained because less power is required on the forward and reverse links. This implies that total system

interference is reduced. As a result, the average system capacity is improved. Also less transmit power from the

mobile results in longer battery life and longer talk time. In a soft handoff, if a mobile receives an up power control

bit from one base station and a down control bit from the second base station, the mobile decreases its transmit

power. The mobile obeys the power down command since a good communications link must have existed to

warrant the command from the second base station.

INTERSYSTEM HANDOFF

Occasionally, a call may be initiated in one cellular system (controlled by one MSO) and enter

another system (controlled by another MSO) before terminating. In some instances, intersystem handoff can

take place; this means that a call handoff can be transferred from one system to a second system so that the call is

continued while the mobile unit enters the second system. The software in the MSO must be modified to apply

this situation. Consider the simple diagram shown in Fig.7. The car travels on a highway and the driver originates a

call in system A. Then the car leaves cell site A of system A and enters cell site B of system B. Cell sites A and B are

controlled by two different MSOs. When the mobile unit signal becomes weak in cell site A, MSO A searches

for a candidate cell site in its system and cannot find one. Then MSO A sends

The handoff request to MSO B through a dedicated line between MSO A and MSO B, and MSO B makes a

complete handoff during the call conversation. This is just a one-point connection case. There are many ways of

implementing intersystem handoffs, depending on the actual circumstances. For instance, if two MSOs are

manufactured by different companies, then compatibility must be determined before implementation of

intersystem handoff can be considered.

Fig.7. Intersystem handoffs

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